The present invention relates to a process for preparing a heteroaromatic aldehyde by catalytic reaction of an alkyl-substituted heteroaromatic compound with molecular oxygen in a gaseous phase. The heteroaromatic aldehyde is a useful compound for various applications such as an raw material for pharmaceutical and agricultural chemicals.
As a process for preparing a heteroaromatic aldehyde by catalytic reaction of an alkyl-substituted heteroaromatic compound with molecular oxygen, for example German Patent Application DD 298234 Specification discloses a process for preparing 4-pyridinecarbaldehyde by catalytic reaction of 4-methylpyridine with molecular oxygen in a gaseous phase with divanadyl pyrophosphate [(VO)2P2O7] as a catalyst.
The present inventors found that, as shown in Comparative Examples, selectivity of 4-pyridinecarbaldehyde was maximum of 60.9% in case of the catalytic reaction of 4-methylpyridine with molecular oxygen in the presence of divanadyl pyrophosphate as the catalyst in a gaseous phase. It was also found that, employing 3-methylpyridine instead of 4-methylpyridine, the selectivity of 3-aldehyde pyridine was also low in case of preparation of 3-pyridinecarbaldehyde by catalytic reaction of 3-methylpyridine with molecular oxygen in a gaseous phase.
Further, according to knowledge of the present inventors, in case of employing divanadyl pyrophosphate as a catalyst, yield of the desired compound (4-pyridinecarbaldehyde, 3-pyridinecarbaldehyde) became extremely low by decreasing an amount of molecular oxygen to increase a concentration of the substrate (4-methylpyridine, 3-methylpyridine) in the reaction materials in order to improve productivity.
Therefore, the processes described above is not satisfactory as the industrial process for preparing a heteroaromatic aldehyde such as pyridinecarbaldehyde, and a process which may prepare a heteroaromatic aldehyde in improved selectivity is desired.
The object of the present invention is to provide a process for solving the problems of the processes in the prior art, namely a process for preparing a heteroaromatic aldehyde by catalytic reaction of an alkyl-substituted heteroaromatic compound with molecular oxygen in a gaseous phase, wherein the heteroaromatic aldehyde is prepared in high selectivity.
The present inventors have found, as a result of effortful studying in order to solve the above described problems, that the heteroaromatic aldehyde may be prepared from an alkyl-substituted heteroaromatic compound in high selectivity, if the heteroaromatic aldehyde is prepared by catalytic reaction of the alkyl-substituted heteroaromatic compound with molecular oxygen in a gaseous phase, wherein the catalyst is an oxide containing vanadium, phosphorus, aluminium and silicon, and a part or all of the oxide is diluted with a solid inert to the reaction.
Namely, the present invention relates to a process for preparing a heteroaromatic aldehyde by catalytic reaction of an alkyl-substituted heteroaromatic compound with molecular oxygen in a gaseous phase in the presence of a catalyst, which comprises employing an oxide containing vanadium, phosphorus, aluminium and silicon as a catalyst, and diluting a part or all of the oxide present as a catalyst layer in a reactor with a solid inert to the reaction.